JP2018026445A - Piezoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric element which improves the signal-to-noise ratio with high sensitivity by suppressing the influence of residual stress of a piezoelectric thin film and eliminating the restriction of an external shape.SOLUTION: Piezoelectric thin films 3a, 3b have a laminated structure, and in each piezoelectric thin film, a plurality of piezoelectric elements are formed having electrodes 4a1, 4a2, 4b1, 4b2, 4c1, 4c2 so as to sandwich a part thereof. Respective piezoelectric elements are arranged so as to be vertically symmetrically superposed. With this configuration, a piezoelectric voltage generated by residual stress and temperature fluctuation is offset mutually by output of the piezoelectric element vertically symmetrically superposed and the signal-to-noise ratio is improved.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a piezoelectric element, and more particularly to a piezoelectric element with high sensitivity and low noise.

  2. Description of the Related Art In recent years, smartphones whose demand has been rapidly expanding are often small-sized, thin-type microphones using MEMS (Micro Electro Mechanical System) technology having high-temperature processing resistance in an assembly solder reflow process. In addition to MEMS microphones, other MEMS elements are rapidly spreading in various fields.

  Most of this type of MEMS element is a capacitive element that detects a vibration displacement of a diaphragm due to an acoustic pressure or the like as a change in capacitance with an opposing fixed plate, converts it into an electric signal, and outputs it. However, improvement in the signal-to-noise ratio of the capacitive element is becoming a limit due to acoustic resistance generated by the flow of air in the gap between the diaphragm and the fixed plate.

  Therefore, attention has been paid to a piezoelectric element that can extract acoustic pressure and the like as a voltage change due to distortion of a single diaphragm formed of a piezoelectric thin film.

  By the way, it is known that the piezoelectric element is output as an unnecessary signal in which the residual stress and temperature fluctuation of the piezoelectric thin film are unnecessary when there is no acoustic pressure or the like, thereby deteriorating the characteristics. Therefore, a technique for releasing residual stress by adopting a cantilever structure in which one end of a piezoelectric thin film is a free end is disclosed (for example, Patent Document 1).

  FIG. 9 shows a cross-sectional view of a piezoelectric element having a cantilever structure. As shown in FIG. 9, piezoelectric thin films 3a and 3b having a multilayer structure are fixed to a silicon substrate 1 serving as a support substrate via an insulating film 2, and the piezoelectric thin film 3a is formed by an electrode 4a and an electrode 4b from above and below. Has a structure sandwiched between the electrode 4b and the electrode 4c. Each of the piezoelectric thin film and the electrode has a rectangular planar shape, and one end is fixed to the silicon substrate 1 and the other end is a free end. The electrodes 4a and 4c are connected to one wiring electrode 5a, and the electrode 4b is connected to another wiring metal 5b.

  In such a piezoelectric element, when the piezoelectric thin film 3a is distorted by receiving an acoustic pressure or the like, polarization occurs in the piezoelectric thin film 3a, and a voltage signal is extracted from the wiring metal 5a connected to the electrode 4a and the wiring metal 5b connected to the electrode 4b. Is possible. Similarly, when the piezoelectric thin film 3b is distorted, polarization occurs therein, and a voltage signal can be extracted from the wiring metal 5a connected to the electrode 4c and the wiring metal 5b connected to the electrode 4b.

  By the way, although the residual stress of the piezoelectric thin film is released by adopting the cantilever structure, as a result, the piezoelectric thin film warps, and the gap between the adjacent piezoelectric thin films (gap between beams G), the side surface of the piezoelectric thin film (beam) and the support substrate The size of the substantial gap increases. The generation of such a gap exceeding the design value reduces acoustic resistance when the piezoelectric element is used as a microphone, leading to characteristic deterioration such as low frequency sensitivity reduction.

  Therefore, in order to solve this problem, the shape of the piezoelectric thin film is a triangle instead of a rectangle, and for example, by arranging so that the apex of each of the four triangles is located at the center, even if the piezoelectric thin film warps A technique is also disclosed in which similar warpage occurs in adjacent piezoelectric thin films, and as a result, the size of the gap between adjacent piezoelectric thin films is not significantly changed (Patent Document 2).

Japanese Patent No. 5707323 Special table 2014-515214 gazette

  In order to prevent characteristic deterioration due to the residual stress of the piezoelectric thin film, the piezoelectric thin film has a triangular shape in the conventional piezoelectric element, and is arranged so that the apexes of the four triangles are gathered at the center to thereby measure the gap of the piezoelectric thin film. It was made possible not to change greatly. However, in order to match the resonance frequency of each triangular beam, it is necessary to combine triangles having the same shape, and there is a problem that the outer shape of the piezoelectric MEMS element is limited to a square and the degree of design freedom is lost. It is an object of the present invention to provide a piezoelectric element that suppresses the influence of residual stress of a piezoelectric thin film and solves the problem that the outer shape is limited, and has high sensitivity and improved signal-to-noise ratio.

  In order to achieve the above object, the invention according to claim 1 of the present application is a piezoelectric element comprising: a piezoelectric thin film having both ends fixed to a support substrate; and a pair of electrodes disposed with the piezoelectric thin film interposed therebetween. Comprises a laminated structure including at least a first piezoelectric thin film and a second piezoelectric thin film, and includes a plurality of sets of the pair of electrodes arranged with a part of the first piezoelectric thin film interposed therebetween, wherein at least the first A piezoelectric element, a second piezoelectric element, and a third piezoelectric element; and a plurality of the pair of electrodes arranged with a part of the second piezoelectric thin film interposed therebetween, and at least a fourth piezoelectric element. An element, a fifth piezoelectric element, and a sixth piezoelectric element are formed, and the first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are connected from one end side to the other end of the both ends. Arranged side by side in order, and 4 piezoelectric elements, the fifth piezoelectric element, and the sixth piezoelectric element are arranged in order from one end side to the other end side of the both ends, and the first piezoelectric element and the fourth piezoelectric element. The piezoelectric element of the element, the second piezoelectric element and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric element is connected in parallel, and the parallel connected piezoelectric elements are in series. And the first piezoelectric element and the fourth piezoelectric element are laminated in a vertically symmetrical manner, and the second piezoelectric element and the fifth piezoelectric element are laminated in a vertically symmetrical manner. The third piezoelectric element and the sixth piezoelectric element are stacked in a vertically symmetrical manner.

  The invention according to claim 2 of the present application is the piezoelectric element according to claim 1, wherein the first to sixth piezoelectric elements are the first piezoelectric element, the second piezoelectric element, and the third piezoelectric element. The fourth piezoelectric element, the fifth piezoelectric element and the sixth piezoelectric element are connected in series, and the two sets of piezoelectric elements connected in series are connected in parallel. Characterize

  The invention according to claim 3 of the present application is the piezoelectric element according to claim 1 or 2, wherein the set of the piezoelectric elements connected in parallel includes the front surface and the back surface of the first piezoelectric thin film or the second piezoelectric thin film. Or it is connected in series by the extension part which continues from the electrode of the said piezoelectric element arrange | positioned between films | membranes, It is characterized by the above-mentioned.

  The invention according to claim 4 of the present application is the piezoelectric element according to any one of claims 1 to 3, wherein when the piezoelectric thin film is bent and displaced by vibration, at least the region divided by the inflection point of the displacement Any of the first piezoelectric element and the fourth piezoelectric element, the second piezoelectric element and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric element, which are stacked in a vertically symmetrical manner. Is arranged.

  The invention according to claim 5 of the present application is the piezoelectric element according to any one of claims 1 to 4, wherein the piezoelectric thin film is a film that vibrates due to an acoustic pressure.

  According to the piezoelectric element of the present invention, the piezoelectric element formed on the first piezoelectric thin film and the piezoelectric element formed on the second piezoelectric thin film are arranged so as to overlap vertically. The piezoelectric voltage generated due to the above is canceled out to reduce the influence of the residual stress of the piezoelectric thin film, and the piezoelectric voltage generated by the first piezoelectric thin film and the piezoelectric voltage generated by the second piezoelectric thin film caused by the acoustic pressure are By superimposing, it is possible to increase the level of the output signal.

  In addition, according to the present invention, the piezoelectric thin film is prevented from being greatly deformed by adopting a doubly-supported beam structure in which both ends of the piezoelectric thin film are fixed to the support substrate, and the shape is not limited to a square, and the degree of freedom in design It is possible to ensure.

  Furthermore, according to the present invention, when the piezoelectric thin film is curved and deformed by vibration, the piezoelectric element formed on the first piezoelectric thin film and the second piezoelectric thin film are formed for each region defined by the inflection point of the displacement. By arranging a pair with the piezoelectric element, the piezoelectric element is separated into the tensile stress area and the compressive stress area generated in the beam extension direction for each divided area, and the voltage signal generated in each area is superimposed By connecting in such a manner, it is possible to efficiently convert it into electric energy and take it out.

  According to the present invention, the connection between the piezoelectric elements can be performed by extending the electrodes of the piezoelectric elements, and the electrical connection can be efficiently performed in that no connection means such as a through hole that affects the displacement of the piezoelectric thin film is required. There is an advantage that it can be converted into energy.

  In particular, when the piezoelectric thin film of the piezoelectric element of the present invention is set to a thickness that vibrates with acoustic pressure and used as an acoustic transducer, high sensitivity and improvement in signal to noise ratio are expected.

It is a top view of the piezoelectric element of the 1st example of the present invention. It is sectional drawing of the piezoelectric element of 1st Example of this invention. It is explanatory drawing when an acoustic pressure signal is applied to a piezoelectric element and a piezoelectric thin film is displaced. It is a graph which shows the film thickness dependence of the piezoelectric thin film which consists of aluminum nitride of a signal noise ratio. It is a graph which shows the relationship between a microphone characteristic and slit width. It is a figure explaining the manufacturing method of the piezoelectric element of 1st Example of this invention. It is a graph which shows the film thickness dependence of the piezoelectric thin film which consists of scandium aluminum nitride of a signal noise ratio. It is sectional drawing of the piezoelectric element of the 2nd Example of this invention. It is explanatory drawing of the conventional piezoelectric MEMS element.

  The piezoelectric element of the present invention has a doubly supported beam structure in which both ends of a piezoelectric thin film are fixed to a support substrate. The piezoelectric thin film has a laminated structure including at least two piezoelectric thin films. Each piezoelectric thin film is formed with a plurality of piezoelectric elements having electrodes arranged so as to sandwich a part thereof, and the piezoelectric elements are connected in parallel or in series. In particular, in the present invention, the piezoelectric elements are arranged so as to overlap vertically. In the present invention, the piezoelectric voltage generated by the residual stress and the temperature fluctuation is canceled by the outputs of the piezoelectric elements that are symmetrically overlapped with each other, and the signal-to-noise ratio is improved. Furthermore, by arranging a set of piezoelectric elements that overlap vertically symmetrically at a predetermined position, a signal can be efficiently extracted. Hereinafter, the case where the piezoelectric element of the present invention is configured as an acoustic transducer will be described in detail.

FIG. 1 is a plan view of a piezoelectric element according to a first embodiment of the invention, and FIG. 2 is a cross-sectional view of the piezoelectric element shown in FIG. As shown in FIG. 2, piezoelectric thin films 3a and 3b are laminated on a silicon substrate 1 serving as a support substrate via an insulating film 2 made of a silicon oxide film (SiO ₂ ). In this embodiment, a slit 6 extending in the horizontal direction of the drawing is formed as shown in FIG. For example, aluminum nitride (AlN) can be used as the piezoelectric thin film, and the crystal orientation (piezoelectric orientation) is formed in the same direction in each of the stacked piezoelectric thin films.

  The structure of the piezoelectric element of this embodiment will be described in detail. Electrodes 4a1 and 4a2 are formed on the back surface side of the piezoelectric thin film 3a, and the electrode 4a1 is connected to the wiring electrode 5a. The electrode 4a2 is not connected to the wiring electrode 5a and other electrodes, and is in a floating state. Electrodes 4b1 and 4b2 are formed on the upper surface side of the piezoelectric thin film 3a and on the lower surface side (corresponding to the space between the films) of the piezoelectric element 3b, and the electrode 4b2 is connected to the wiring electrode 5b. The electrode 4b1 is not connected to the wiring electrode 5b and other electrodes and is in a floating state. Furthermore, an electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the piezoelectric thin film 3b. The electrode 4c1 is connected to the wiring electrode 5a, and the electrode 4c2 is not connected to the wiring electrode 5a or other electrodes. , Floating. The electrode can be formed of a metal thin film such as molybdenum (Mo), platinum (Pt), titanium (Ti), iridium (Ir), or ruthenium (Ru).

  With this configuration, the piezoelectric element C1 (corresponding to the first piezoelectric element) is formed in a region where the electrode 4a1, the piezoelectric thin film 3a (corresponding to the first piezoelectric thin film) and the electrode 4b1 overlap. Similarly, in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b1 overlap, the piezoelectric element C2 (corresponding to the second piezoelectric element), and in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap, the piezoelectric element C3 (third piezoelectric element). In the region where the electrode 4c1, the piezoelectric thin film 3b (corresponding to the second piezoelectric thin film) and the electrode 4b1 overlap, the electrode 4c2, the piezoelectric thin film 3b and the electrode 4b1 The piezoelectric element C5 (corresponding to the fifth piezoelectric element) and the piezoelectric element C6 (corresponding to the sixth piezoelectric element) are formed in the area where the electrode 4c2, the piezoelectric thin film 3b and the electrode 4b2 overlap.

  As a result, the first piezoelectric element C1 and the fourth piezoelectric element C4 are connected in parallel, the second piezoelectric element C2 and the third piezoelectric element C3 are connected in series, the fifth piezoelectric element C5 and the sixth piezoelectric element. The element C6 is connected in series with the series connection, and the piezoelectric elements connected in parallel are connected in series between the wiring electrode 5a and the wiring metal 5b.

  Here, for example, the first piezoelectric element C1 and the second piezoelectric element C2 use the electrode 4b1 constituting the piezoelectric element in common, so that the electrodes 4b1 that do not overlap with the opposing electrodes (electrodes 4a1 and 4a2 respectively). Are connected by the region (corresponding to the extension). Similarly, the fourth piezoelectric element C4 and the fifth piezoelectric element C5 share the electrode 4b1 that constitutes the piezoelectric element, so that the region of the electrode 4b1 that does not overlap with the opposing electrodes (electrodes 4c1 and 4c2 respectively). (Corresponding to the extension). Further, the second piezoelectric element C2 and the third piezoelectric element C3 are provided by an electrode 4a2, and the fifth piezoelectric element C5 and the sixth piezoelectric element C6 are provided by an electrode 4c2 so that the electrodes 4a2 (not overlapping with the electrodes facing each other) The electrode 4c2 region (corresponding to the extension). Such connection eliminates the need to form connection means that affect the displacement of the piezoelectric thin film such as a through hole in the piezoelectric thin film.

  As is apparent from FIG. 2, the first piezoelectric element C1 and the fourth piezoelectric element C4, the second piezoelectric element C2 and the fifth piezoelectric element C5, and the third piezoelectric element C3 and the sixth piezoelectric element C6. Is vertically symmetric with respect to a plane passing through the centers of the electrodes 4b1 and 4b2 in the thickness direction at least in the region where each piezoelectric element is formed.

  A part of the back surface of the silicon substrate 1 is removed to form a hole 7, and the electrodes 4 a 1, 4 a 2 and the piezoelectric thin film 3 a are exposed in the hole 7. The holes 7 communicate with the surface side of the silicon substrate 1 through the slits 6 shown in FIG.

  With this configuration, the piezoelectric element of this embodiment has a double-supported beam structure in which a plurality of electrode pairs are formed on a piezoelectric thin film supported at both ends by a silicon substrate 1 (support substrate).

  When the piezoelectric element of the present invention is configured as an acoustic transducer, acoustic pressure is applied from the holes 7 formed in the silicon substrate 1. The beam structure including the piezoelectric thin film subjected to the acoustic pressure is curved and displaced upward. As a result, tensile stress and compressive stress are generated in the aluminum nitride constituting the piezoelectric thin film.

  FIG. 3 shows an example in which an acoustic pressure signal is applied and the piezoelectric thin film is displaced. In this case, two inflection points are generated and divided into three regions depending on the direction of stress on the piezoelectric thin film. For example, the region 1 and the region 3 are bent and displaced downward, and a tensile stress is generated in the first piezoelectric thin film 3a and a tensile stress is generated in the second piezoelectric thin film 3b. On the other hand, the region 2 is curved and displaced upwardly, and compressive stress is generated in the first piezoelectric thin film 3a and tensile stress is generated in the second piezoelectric thin film 3b.

  By the way, as shown in FIG. 2, the piezoelectric element of this embodiment includes a piezoelectric element C1 and a piezoelectric element C4, a series connection of the piezoelectric element C2 and the piezoelectric element C3, and a series connection of the piezoelectric element C5 and the piezoelectric element C6. These are connected in parallel and have a vertically symmetrical structure. For this reason, the voltages generated in each of the regions 1 to 3 have opposite polarities and the same value, so that the in-phase voltage caused by the residual stress and temperature fluctuation is canceled out.

As a result, the output signal (voltage) of each region based on the application of the acoustic pressure signal is superimposed and added without including a signal due to residual stress or temperature fluctuation, and the output voltage ( _Ac ) for the acoustic pressure (P _a ) It is possible to increase the sensitivity as an acoustic transducer defined by the ratio (V _out / P _a ) of V _out ).

The size of each electrode is preferably optimized from the viewpoint of maximizing the signal to noise ratio. This wiring electrodes 5a, the capacity of the equivalent capacitor as seen from 5b when the C _out, of the electrodes so as to maximize the energy stored in the equivalent capacitor ^{_{(C out · V out 2/}} 2) Decide the size.

  Specifically, a design example of the dimensions in the case of a rectangular cantilever beam, the film thickness of each piezoelectric thin film, and the size of the electrode is shown. For example, it is assumed that the input signal is human voice and the resonance frequency of the both-end supported beam is 20 kHz. In addition, the plane dimensions are assumed to be mounted on an electronic device such as a smartphone. The length of the doubly supported beam (corresponding to the length of the slit in FIG. 1) is 0.7 mm, and the width (up and down in FIG. 1) is 1.4 mm. The thicknesses of the piezoelectric thin films 3a and 3b made of aluminum nitride are both 0.5 μm, and the thicknesses of the electrodes 4a1 to 4c1 and 4a2 to 4c2 made of molybdenum are both 0.1 μm. The extension lengths of the electrodes 4a1, 4c1 and 4b2 from the support ends (ends of the air holes 7) are both 90 μm, and the lengths from the support ends of the electrodes 4b1, 4a2 and 4c2 to the electrode ends are both 500 μm. The width of the slit 6 is 1 μm.

The thickness of the piezoelectric thin films 3a and 3b and the length of the cantilever beam (slit) can be determined as follows. FIG. 4 is a graph showing the dependence of the signal-to-noise ratio on the film thickness of a piezoelectric thin film made of aluminum nitride. The resonance frequency was constant (20 kHz), the beam width was 1.4 mm, and the slit width was 1 μm. In consideration of the fact that it is limited by the size of the mounting case when mounted on an electronic device such as a smartphone, the volume of the holes is set to a relatively small value of 3 mm ² . In addition to the piezoelectric element (acoustic transducer) of the present invention, an amplifier circuit for processing the output signal of the piezoelectric element needs to be mounted in a small housing such as a smartphone. Therefore, considering the allowable chip size of the piezoelectric element, the following examination was performed when the length of the beam was 0.6 mm, 0.7 mm, and 0.8 mm.

  As shown in FIG. 4, when the thickness of each piezoelectric thin film made of aluminum nitride is made thinner than 0.4 μm, the signal-to-noise ratio is slightly improved. However, since the substantial acoustoelectric conversion coefficient is limited due to acoustic compliance of the holes, it can be seen that the improvement effect due to the thinning is not remarkable and the dependence on the beam length is not large. On the contrary, when the thickness of the piezoelectric thin film is increased, the signal-to-noise ratio is drastically lowered. In addition, the dependence on the length of the beam becomes significant. In other words, in order to keep the resonance frequency constant, it is necessary to increase the length of the beam when the thickness of the piezoelectric thin film is increased. In the example shown in FIG. 4, when the thickness of the piezoelectric thin film is 0.5 μm, the length of the beam is 0.7 mm, and when the thickness of the piezoelectric thin film is 0.6 μm, the length of the beam is It can be seen that 0.8 mm is preferable.

Next, the slit width will be described. FIG. 5 is a graph showing the relationship between the reduction rate of the sensitivity at 100 Hz and the slit width with reference to the sensitivity of 1 kHz, which is one of the main characteristics of the microphone. In the same manner as described above, the resonance frequency of the both-end supported beam is 20 kHz, the length of the beam is 0.7 mm, the width is 1.4 mm, and the volume of the holes is 3 mm ² . When the thickness of the piezoelectric thin film made of aluminum nitride was 0.4 μm, 0.5 μm, and 0.6 μm, the relationship with the slit width was examined.

  As shown in FIG. 5, almost no dependence on the thickness of the piezoelectric thin film is observed, and when the slit width exceeds 1 μm, the sensitivity decrease becomes significant at 100 Hz. When the frequency is lowered below 100 Hz, this sensitivity reduction becomes more serious. That is, it can be seen that the slit width needs to be 1 μm or less in order to suppress the decrease in sensitivity at a low frequency.

  As described above, in order to increase the sensitivity, the length of the doubly supported beam, the thickness of the piezoelectric thin film, the slit width, and the like may be appropriately adjusted and set.

The piezoelectric element of the present invention can be formed using a normal method for manufacturing a semiconductor device. FIG. 6 is an explanatory diagram of a method of manufacturing the piezoelectric element of this example. First, an insulating film 2 made of a silicon oxide film (SiO ₂ ) is formed on the silicon substrate 1 by a thermal oxidation method. A molybdenum (Mo) film having a thickness of 0.1 μm is stacked on the insulating film 2 by a sputtering method, and patterning is performed by an ordinary lithographic method, thereby forming electrodes 4a1 and 4a2 (FIG. 6a).

  Thereafter, an aluminum nitride film having a thickness of 0.5 μm is laminated on the entire surface by sputtering to form a piezoelectric thin film 3a corresponding to the first piezoelectric thin film. Thereafter, a molybdenum (Mo) film having a thickness of 0.1 μm is laminated on the piezoelectric thin film 3a by sputtering, and patterning is performed by an ordinary lithographic method, thereby forming the electrodes 4b1 and 4b2. Further, an aluminum nitride film having a thickness of 0.5 μm is laminated on the entire surface by sputtering to form a piezoelectric thin film 3b corresponding to the second piezoelectric thin film. A molybdenum (Mo) film having a thickness of 0.1 μm is laminated by sputtering, and patterned by an ordinary lithographic method to form electrodes 4c1 and 4c2 (FIG. 6b).

A part of the piezoelectric thin films 3a and 3b is removed by etching to form a wiring electrode 5a connected to the electrodes 4a1 and 4c1 and a wiring electrode 5b connected to the electrode 4b2. The wiring electrodes 5a and 5b are made of aluminum and can be formed by a normal lithographic method (FIG. 6c).
When a part of the piezoelectric thin films 3a and 3b is removed by etching, a part of the piezoelectric thin films 3a and 3b corresponding to the slit 6 shown in FIG. 1 is also removed by etching to form a recess, and the insulating film 2 is formed on the bottom thereof. Leave it exposed.

  Finally, a part of the back surface of the silicon substrate 1 is removed by a dry etching method, and a part of the exposed insulating film 2 is also removed to form a hole 7. A part of the electrode 4a1, the electrode 4b1, and the piezoelectric thin film 3a previously formed is exposed in the hole 7. By forming the holes 7, the insulating film 2 exposed at the bottom of the recess formed to form the slit 6 is also removed, and the slit 6 is formed through the front surface side and the back surface side of the piezoelectric thin film. Due to the slit 6, the piezoelectric thin film has a double-supported beam structure (FIG. 6d).

Although the piezoelectric element and the manufacturing method thereof according to the present embodiment have been described above, it is needless to say that the present invention is not limited to aluminum nitride as the piezoelectric thin film. Table 1 shows the effects of piezoelectric microphones on typical piezoelectric materials such as aluminum nitride, scandium aluminum nitride (Al _1-x Sc _x N), zinc oxide (ZnO), and lead zirconate titanate (PZT). 3 is a table comparing material constants such as Young's modulus and lateral piezoelectric strain coefficient.

The figure of merit (FOM) corresponding to the signal-to-noise ratio shown in Table 1 is represented by the ratio of the coupling coefficient (k ₃₁ ² ) and the loss angle (tan δ). The larger the value, the more nearly proportional to the value. An improvement in signal to noise ratio can be expected. As shown in Table 1, it can be seen that aluminum nitride is 6 to 40 times larger in performance index than zinc oxide and lead zirconate titanate, and is suitable for piezoelectric transducers. Also, scandium aluminum nitride (Al _1-x Sc _x N) obtained by adding scandium to aluminum nitride is known to have a higher transverse piezoelectric strain coefficient than aluminum nitride. For example, the ratio of scandium was set to 35%. In this case, it can be expected that the figure of merit is improved about 7 times that of aluminum nitride.

FIG. 7 is a graph showing the dependence of the signal-to-noise ratio on the film thickness of a piezoelectric thin film made of scandium aluminum nitride (Al _1-x Sc _x N: x = 0.35). When compared with the graph showing the film thickness dependence of the piezoelectric thin film made of aluminum nitride shown in FIG. 4, it can be seen that an improvement in signal-to-noise ratio of about 8 dB corresponding to 7 times the figure of merit can be expected. In addition, about the film thickness dependence, it has also confirmed that the same tendency was shown irrespective of the kind of piezoelectric thin film. Specifically, when the thickness of the piezoelectric thin film is 0.4 μm, the length of the beam is 0.6 mm, and when the thickness of the piezoelectric thin film is 0.6 μm, the length of the beam is 0.00 mm. When the thickness of the piezoelectric thin film is 7 μm and 7 μm, it is understood that the length of the beam is preferably 0.8 mm.

  Next, a second embodiment of the present invention will be described. FIG. 8 is a sectional view of a piezoelectric element according to a second embodiment of the present invention. Compared with the piezoelectric element shown in FIG. 2 described above, it differs in that it includes a dielectric film 8, an electrode 4d1, an electrode 4d2, and the like.

That is, as shown in FIG. 8, a piezoelectric thin film 3a, a dielectric film 8 and a piezoelectric thin film 3b are laminated on an insulating film 2 made of a silicon oxide film (SiO ₂ ) on a silicon substrate 1 serving as a support substrate. Has been. In this embodiment, in order to have a double-supported beam structure, the slit 6 extending in the lateral direction of the drawing is formed as described with reference to FIG. For example, aluminum nitride (AlN) can be used for the piezoelectric thin film, and the crystal orientation (piezoelectric orientation) is formed in the same direction.

  In the piezoelectric element of this embodiment, an electrode 4a1 and an electrode 4a2 are formed on the back side of the piezoelectric thin film 3a, and the electrode 4a1 is connected to the wiring electrode 5a. The electrode 4a2 is not connected to the wiring electrode 5a and other electrodes, and is in a floating state. An electrode 4b1 and an electrode 4b2 are formed on the upper surface side of the piezoelectric thin film 3a, and the electrode 4b2 is connected to the wiring electrode 5b. The electrode 4b1 is not connected to the wiring electrode 5b and other electrodes and is in a floating state.

  A dielectric film 8 is laminated on the piezoelectric thin film 3a and the electrodes 4b1, 4b2, and an electrode 4d1 and an electrode 4d2 are formed on the dielectric film 8. The electrodes 4d1 and 4d2 have the same shape as the previously formed electrodes 4b1 and 4b2. The electrode 4d2 is connected to the wiring electrode 5b, and the electrode 4d1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state.

  Specifically, an electrode 4d1 and an electrode 4d2 are formed on the upper surface side of the dielectric film 8 and on the lower surface side (corresponding to the space between the films) of the piezoelectric element 3b. The electrode 4d2 is connected to the wiring electrode 5b. ing. The electrode 4d1 is not connected to the wiring electrode 5b and other electrodes and is in a floating state. Furthermore, an electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the piezoelectric thin film 3b. The electrode 4c1 is connected to the wiring electrode 5a, and the electrode 4c2 is not connected to the wiring electrode 5a or other electrodes. , Floating. The electrode can be formed of a metal thin film such as molybdenum (Mo), platinum (Pt), titanium (Ti), iridium (Ir), or ruthenium (Ru).

  With this configuration, the piezoelectric element C1 (corresponding to the first piezoelectric element) is formed in a region where the electrode 4a1, the piezoelectric thin film 3a (corresponding to the first piezoelectric thin film) and the electrode 4b1 overlap. Similarly, in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap, the piezoelectric element C2 (corresponding to the second piezoelectric element), and in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap, the piezoelectric element C3 (third In the region where the electrode 4c1, the piezoelectric thin film 3b (corresponding to the second piezoelectric thin film) and the electrode 4d1 overlap, the piezoelectric element C4 (corresponding to the fourth piezoelectric element) is the electrode 4c2, and the piezoelectric thin film 3b. The piezoelectric element C5 (corresponding to the fifth piezoelectric element) is formed in the area where the electrode 4d1 overlaps, and the piezoelectric element C6 (corresponding to the sixth piezoelectric element) is formed in the area where the electrode 4c2, the piezoelectric thin film 3b and the electrode 4d2 overlap. Is done.

  As a result, the first piezoelectric element C1, the second piezoelectric element C2, and the third piezoelectric element C3 are connected in series between the wiring electrode 5a and the wiring metal 5b. Similarly, the fourth piezoelectric element C4, the fifth piezoelectric element C5, and the sixth piezoelectric element C6 are connected in series between the wiring electrode 5a and the wiring metal 5b. Further, a set of these piezoelectric elements connected in series is connected in parallel.

  Even if formed in this way, as is apparent from FIG. 8, the first piezoelectric element C1 and the fourth piezoelectric element C4, the second piezoelectric element C2 and the fifth piezoelectric element C5, and the third piezoelectric element C3. The sixth piezoelectric element C6 has a structure that is vertically symmetrical with respect to a plane that passes through the center of the dielectric film 8 in the thickness direction at least in a region where each piezoelectric element is formed.

  On the back surface side of the silicon substrate 1, a hole 7 from which a part thereof is removed is formed, and the electrodes 4a1, 4a2 and the piezoelectric thin film 3a are exposed in the hole 7. The holes 7 communicate with the surface side of the silicon substrate 1 through the slits 6 shown in FIG.

  With this configuration, the piezoelectric element according to the present embodiment can also have a double-supported beam structure in which a plurality of electrode pairs are formed on a piezoelectric thin film supported at both ends by a silicon substrate 1 (support substrate).

  When the piezoelectric element of this embodiment is configured as an acoustic transducer, an acoustic pressure is applied from the holes 7 formed in the silicon substrate 1. The beam structure including the piezoelectric thin film subjected to the acoustic pressure is curved and displaced upward. As a result, tensile stress and compressive stress are generated in the aluminum nitride constituting the piezoelectric thin film.

  However, as in the first embodiment, the piezoelectric element C1 and the piezoelectric element C4, the piezoelectric element C2 and the piezoelectric element C5, and the piezoelectric element C3 and the piezoelectric element C6 have a vertically symmetrical structure, respectively. Similarly to the case described, the voltages generated in the respective regions have opposite polarities and the same value, so that the in-phase voltage caused by the residual stress and temperature fluctuation can be canceled out.

As a result, the output signal (voltage) of each region based on the application of the acoustic pressure signal is superimposed and added without including a signal due to residual stress or temperature fluctuation, and the output voltage ( _Ac ) for the acoustic pressure (P _a ) It is possible to increase the sensitivity as an acoustic transducer defined by the ratio (V _out / P _a ) of V _out ). In particular, the piezoelectric signal is extracted from the uppermost piezoelectric thin film 3b and the lowermost piezoelectric thin film 3a of the laminated structure, and the piezoelectric signal is extracted from a portion where the stress is relatively large, so that the signal-to-noise ratio is expected to be further improved. Is done.

The size of each electrode is preferably optimized from the viewpoint of maximizing the signal to noise ratio. This wiring electrodes 5a, the capacity of the equivalent capacitor as seen from 5b when the C _out, of the electrodes so as to maximize the energy stored in the equivalent capacitor ^{_{(C out · V out 2/}} 2) The same applies to the determination of the size. In addition, the thickness and material of the dielectric film may be appropriately selected in order to obtain desired characteristics. The dielectric film may be aluminum nitride. When three layers of aluminum nitride are stacked, the thickness of each layer may be 0.33 μm.

  When bending inflection is caused by vibration of the piezoelectric thin film, if the inflection point of the displacement is 2 or more, the present invention is not limited to the above embodiment, and the number of piezoelectric elements is increased for each region defined by the inflection point. Alternatively, a plurality of elements may be arranged in each region.

  By the way, when the piezoelectric element as in the present invention is used as an acoustic transducer, an amplification circuit for processing a signal output from the piezoelectric element is required. In general, an integrated circuit is formed on another silicon substrate, and each is mounted on a mounting substrate as a separate element. The piezoelectric element of the present invention has no problem even if an element for signal processing, for example, is formed on the support substrate. In that case, the piezoelectric thin film may be used as an interlayer insulating film and the electrode may be used as a wiring metal.

1: silicon substrate, 2: insulating film, 3a, 3b: piezoelectric thin film, 4a, 4b, 4c, 4d: electrode, 5a, 5b: wiring electrode, 6: slit, 7: hole, 8: dielectric film

Claims (5)

In a piezoelectric element comprising a piezoelectric thin film having both ends fixed to a support substrate, and a pair of electrodes arranged with the piezoelectric thin film interposed therebetween,
The piezoelectric thin film has a laminated structure including at least a first piezoelectric thin film and a second piezoelectric thin film;
A plurality of the pair of electrodes arranged with a part of the first piezoelectric thin film interposed therebetween, wherein at least a first piezoelectric element, a second piezoelectric element, and a third piezoelectric element are formed;
A plurality of the pair of electrodes arranged with a part of the second piezoelectric thin film interposed therebetween, and at least a fourth piezoelectric element, a fifth piezoelectric element, and a sixth piezoelectric element are formed;
The first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are arranged in order from one end side to the other end side of the both ends;
The fourth piezoelectric element, the fifth piezoelectric element, and the sixth piezoelectric element are arranged in order from one end side to the other end side of the both ends;
The piezoelectric elements of the first piezoelectric element and the fourth piezoelectric element, the second piezoelectric element and the fifth piezoelectric element, or the combination of the third piezoelectric element and the sixth piezoelectric element are arranged in parallel. The piezoelectric elements connected in parallel are connected in series;
The first piezoelectric element and the fourth piezoelectric element are laminated in a vertically symmetrical manner, the second piezoelectric element and the fifth piezoelectric element are laminated in a vertically symmetrical manner, and the third piezoelectric element And the sixth piezoelectric element are laminated in a vertically symmetrical manner. The piezoelectric element according to claim 1, wherein
The first to sixth piezoelectric elements include the first piezoelectric element, the second piezoelectric element, and the third piezoelectric element connected in series, and the fourth piezoelectric element and the fifth piezoelectric element. And the sixth piezoelectric element are connected in series, and the two sets of piezoelectric elements connected in series are connected in parallel. The piezoelectric element according to claim 1 or 2,
The set of the piezoelectric elements connected in parallel is connected in series by an extension continuous from the electrodes of the piezoelectric elements arranged between the front surface, the back surface, or the film of the first piezoelectric thin film or the second piezoelectric thin film. A piezoelectric element characterized by comprising: The piezoelectric element according to any one of claims 1 to 3,
When the piezoelectric thin film is bent and displaced by vibration, at least the first piezoelectric element and the fourth piezoelectric element, which are stacked in a vertically symmetrical manner, for each region defined by the inflection point of the displacement, One of the second piezoelectric element and the fifth piezoelectric element or the third piezoelectric element and the sixth piezoelectric element is arranged. The piezoelectric element according to any one of claims 1 to 4,
The piezoelectric element is a film that vibrates by acoustic pressure.

Images